1. Field of the Invention
The present invention relates to an apparatus for exposing an image of an original film on a photosensitive material in a pre-press process of a photomechanical printing process, and particularly relates to a vacuum contact printing apparatus for subjecting a photosensitive material such as a photosensitive film or a PS (presensitized) plate to exposure while a flexible overlay sheet absorbed under vacuum on a table where the photosensitive material is placed holds the photosensitive material and an original film placed under the sheet in intimate contact. The present invention further relates to a method for exposure by holding an original and a photosensitive material in intimate contact utilizing vacuum.
2. Description of the Background Art
FIG. 1 is a plan view of a conventional apparatus. The conventional apparatus comprises: a base frame 12 having a transparent plate 10 such as a glass plate on its upper surface; a pin bar 20 provided with two register pins 18 for defining a reference position for registration between an original film 14 and a photosensitive material 16; a sheet roller 22 for winding or spreading an overlay sheet 24 from its free end, the opposite end of the overlay sheet 24 being fixed to one side of the upper surface of the base frame 12 by means of a fixing member 23; evacuating means 28 for removing air from between the transparent plate 10 and the overlay sheet 24 when the overlay sheet 24 is spread over the transparent plate 10; and a light source (not shown) for exposure, provided under the transparent plate 10. An evacuating groove 26 communicating with the evacuating means 28 is formed around the transparent plate 10. The overlay sheet 24 has sufficient width and length to entirely cover the evacuating groove 26 when it is spread over the base frame 12. The overlay sheet 24 is a rubber sheet which flexibly changes its form in vacuum contact with the base frame, making it possible to tightly hold the original film 14 and the photosensitive material 16. Further, the rubber overlay sheet 24 has sufficient durability.
Referring to FIGS. 1 and 2, the operation of the conventional apparatus will now be described. The overlay sheet 24 is wound on the sheet roller 22 beforehand. The evacuating means 28 is not in operation. The operator places and registers the original film 14 and the photosensitive material 16 on the transparent plate 10 by using the register pins 18 provided on the pin bar 20. The registration is carried out by inserting the register pins 18 into register holes formed in the original film 14 and the photosensitive material 16.
When the sheet roller 22 rotates and moves toward the direction of the arrow B, the overlay sheet 24 spreads over the original film 14 and the photosensitive material 16. After the overlay sheet 24 is sufficiently spread, the evacuating means 28 operates and discharges the air through the evacuating groove 26. As a result, the overlay sheet 24 is absorbed on the transparent plate 10, whereby the original film 14 and the photosensitive material 16 placed thereunder are held in close contact.
Light L for exposure is applied from below. As a result, a latent image corresponding to an image on the original film 14 is formed on the photosensitive material 16. Subsequently, the evacuating means 28 stops and the overlay sheet 24 is released from the vacuum. After the sheet roller 22 winds up the overlay sheet 24, the original film 14 and the photosensitive material 16 are taken out from the transparent plate 10.
As described above, the overlay sheet 24 is made of rubber. When absorbed on the transparent plate 10 under vacuum, the overlay sheet 24 stretches suitably, thereby making the intimate contact between the original film 14 and the photosensitive material 16 more reliable.
The apparatus for spreading the overlay sheet 24 by using the sheet roller 22 has various advantages as described below. One of the advantages is that it is easy to automatically carry out the operation of spreading the overlay sheet 24. Another advantage is that the air between the overlay sheet 24 and the transparent plate 10 is removed smoothly by means of a pressing roller which is provided behind the sheet roller 22 (in the direction of the fixing member 23), though not shown in FIGS. 1 and 2, and presses the overlay sheet 24 while moving together with the sheet roller 22. A further advantage is that the original film 14 and the photosensitive material 16 are hardly misaligned due to air current because there is not large movement of air in spreading the overlay sheet.
Normally, in color printing, four printing processes using cyan, magenta, yellow and black inks are carried out. For the printing by using the inks of those colors, it is necessary to prepare color separation films corresponding to those colors and to print images of those color separation films onto PS plates and so on. Those images are formed by fine dots for convenience of printing. Accordingly, in order to print the images on other photosensitive material, it is necessary to prevent disagreement of the images and error in size as far as possible. Therefore, for the above described reasons, the original film and the photosensitive material registered by the register pins are maintained in intimate contact under vacuum.
However, the conventional apparatus has disadvantages as described below. Prior to the printing of the images, the overlay sheet 24 is spread over the photosensitive material 16. Since the overlay sheet 24 is a relatively thick rubber sheet, force F pushing back the photosensitive material 16 toward the direction of the arrows b is applied to the contact face 24a between the overlay sheet 24 and the photosensitive material 16. The causes for this phenomenon are considered to be as follows.
When the overlay sheet 24 is wound around the sheet roller 22, stress due to tension occurs in the outer circumferential portions of the overlay sheet 24. Stress due to contraction occurs in the inner circumferential portions of the overlay sheet 24. When the overlay sheet 24 spreads from the sheet roller 22, the outer circumferential portion of the overlay sheet 24 contracts and returns from the state subjected to the tension stress to the normal state. This change occurs in the portion of the overlay sheet 24 contacting the photosensitive material 16. Consequently, frictional force F of the overlay sheet 24 is applied to the upper surface of the photosensitive material 16 in the direction of the arrows b.
This force F sometimes causes the photosensitive material 16 to move in the direction of the arrows b with respect to the original film 14. As a result, registration is incorrect. Accordingly, the quality of the final color printing is lowered.